I heard the term "worker thread" and other types of threads such as "system thread" and "user thread".
For me a thread is nothing but a lite weight process and multi-threaded design is chosen for a software development when there are multiple task in the application/system and those tasks can be executed independently.
I want to know when these different terms are used like worker thread, user thread and system thread ?

Here pthread_mutex_t lock ; is also a global variable shared to threads. Accessing of this global variable (lock) will it be shame as accessing of other global variables ? If same, then don't we face same problem what we will face for other global variables ? if not ,how this is discriminated from other global variables ?